Amphiphilic molecules exhibit distinct regions of polar and nonpolar character. These regions can result from substitution of hydrophobic and hydrophilic substituents into specific and distinct regions of conformationally defined molecules. Alternately a conformationally flexible molecule or macromolecule can adopt an ordered structure in which the hydrophobic and hydrophilic substituents on the molecule segregate to different areas or faces of the molecule. Commonly occurring amphiphilic molecules include surfactants, soaps, detergents, peptides, proteins and copolymers. These molecules have the capacity to self-assemble in appropriate solvents or at interfaces to form a variety of amphiphilic structures. The size and shape of these structures varies with the specific composition of the amphiphilic molecule and solvent conditions such as pH, ionic strength and temperature.
Amphiphilic peptides with unique broad-spectrum antimicrobial properties have been isolated from a variety of natural sources including plants, frogs, moths, silk worms, pigs and humans (H. G. Boman Immunol Rev. 2000 173:5–16; R. E. Hancock and R. Lehrer, Trends Biotechnol. 1998 16:82–88). These compounds include the magainin 1 (1) and dermaseptin S1 (2) isolated from the skin of frogs and the cecropin A (3) isolated from the cecropia moth. These naturally occurring compounds have broad-spectrum antibacterial activity and they do not appear prone to the development of bacterial resistance. These compounds are relatively low molecular weight peptides that have a propensity to adopt α-helical conformation in hydrophobic media or near a hydrophobic surface and as a result are facially amphiphilic (i.e., one-third to two-thirds of the cylinder generated by the helical peptide has hydrophobic side chains while the
GIGKFLHSAGKFGKAFVGEIMKS-CO2H(1) ALWKTMLKKLGTMALHAGKAALGAAADTISQGTQ-CO2H(2) KWKLFKKIEKVGQNIRDGIIKAGPAVAVVGQATQIAK-NH2(3) RGGRLCYCRRRFCVCVGR-NH2(4)remainder has hydrophilic side chains. These hydrophilic side chains are primarily positively-charged at neutral pH. Hydrophobic amino acids compose 40–60% of the total number of residues in most anti-microbial peptides. The selectivity of the amphiphilic peptides (e.g. for bacteria vs. human erythrocytes) depends on the overall hydrophobicity. The biological activity of thee compounds depend on the ratio of charged (c) to hydrophobic (h) residues. When the ratio is varied from 1:1 (c:h) to 1:2 (c:h) peptides with more hydrophobic residues tend to be more active toward erythrocyte membranes. The physiochemical properties rather than the presence of particular amino acids or the tertiary structure of the side chains. Related peptides have been isolated from mammals and these anti-microbial peptides have been suggested to be an important component of the innate immune response. (Gennaro, R. et al. Biopoylmers (Peptide Science) 2000, 55, 31).
These observations recently have been extended to peptides (β-peptides) comprised of β-amino acids. These non-natural polypeptide mimetics also are capable of adopting stable α-helical and β-sheet structures although the precise geometries of these structure are different form those generated by α-amino acid oligomers. However, appropriate positioning of hydrophobic and hydrophilic residues results in amphiphilic conformations with similar antimicrobial properties. This further confirms the importance of repeating periodicity of hydrophobic and hydrophilic groups vis-à-vis the precise amino acid sequence in producing facial amphiphilic antimicrobial compounds. (D. Seebach and J. L. Matthews, Chem Commun. 1997 2105; Hamuro, Y., Schneider, J. P., DeGrado, W. F., J. Am. Chem. Soc. 1999, 121, 12200–12201; D. H. Appella et al., J. Am. Chem. Soc., 1999 121, 2309).
Secondary structures other than helices may also give rise to amphiphilic compounds. The protegrins (4) are a related series of anti-microbial peptides. (J. Chen et al., Biopolymers (Peptide Science), 2000 55 88) The presence of a pair of disulfide bonds between Cys6–Cys15 and Cys8–Cys13 results in a monomeric amphiphilic anti-parallel β-sheet formed by the chain termini and linked by a β-turn. The amphiphilic β-sheet conformation is essential for anti-microbial activity against both gram-positive and gram-negative bacteria.
The data related to anti-microbial peptides suggests that facial amphiphilicity, the alignment of polar (hydrophilic) and nonpolar (hydrophobic) side chains on opposite faces of a secondary structural element formed by the peptide backbone, and not amino acid sequence, any particular secondary/tertiary structure, chirality or receptor specificity is responsible for their biological activity.
Suitably substituted polymers which lack polyamide linkages also are capable of adopting amphiphilic conformations. Solid phase chemistry technology was utilized to synthesize a class of meta substituted phenylacetylenes that fold into helical structures in appropriate solvents (J. C. Nelson et al., Science 1997 277:1793–96; R. B. Prince et al., Angew. Chem. Int. Ed. 2000 39:228–231). These molecules contain an all hydrocarbon backbone with ethylene oxide side chains such that when exposed to a polar solvent (acetonitrile), the backbone would collapse to minimize its contact with this polar solvent. As a result of the meta substitution, the preferred folded conformation is helical. This helical folding is attributed to a “solvophobic” energy term; although, the importance of favorable π—π aromatic interactions in the folded state are also likely to be important. Furthermore, addition of a less polar solvent (CHCl3) results in an unfolding of the helical structure demonstrating that this folding is reversible.
Regioregular polythiophenes (5 and 6) have been shown to adopt amphiphilic conformations in highly ordered π-stacked arrays with hydrophobic side chains on one side of the array and hydrophilic side chains on the other side. These polymers form thin films useful in the construction of nanocircuits. (Bjørnholm et al., J. Am. Chem. Soc., 1998 120, 7643) These materials would be facially amphiphilic as defined herein; however, no biological properties have reported for these compounds.

Antimicrobial peptides have been incorporated onto surfaces or bulk materials, with some retention of antimicrobial properties. Haynie and co-workers at DuPont have investigated the activity of Antibacterial peptides have been covalently attached to solid surfaces (S. L. Haynie et al., Antimicrob Agents Chemother, 1995 39:301–7; S. Margel et al., J Biomed Mater Res, 1993, 27:1463–76). A variety of natural and de novo designed peptides were synthesized and tested for activity while still attached to the solid support. The activity of the peptides decreased when attached to the solid support although the peptides retained their broad spectrum of activity. For example, a de novo designed peptide referred to as E14LKK has a MBC (minimum bactericidal activity) of 31 μg/ml in solution as opposed to 1.5 mg/ml when attached to a solid phase bead. The peptides were attached to the resin with a 2 to 6-carbon alkyl linker. The porosity of Pepsyn K, the resin used in the synthesis, is small (0.1 to 0.2 μm) compared to the bacteria, so the microbes may be unable to penetrate into the interior of the resin. Thus the great majority of the peptide would not be available for binding to cells. The antimicrobial activity did not arise from a soluble component; no leached or hydrolyzed peptide was observed and the soluble extracts were inactive. These studies indicate quite convincingly that antimicrobial peptides retain their activity even when attached to a solid support. However, there is a need to optimize the presentation of the peptides to increase their potency.
Other antimicrobial polymeric materials have been reported which contain chemical functionality known to be antimicrobial (J. C. Tiller et al., Proc Natl Acad Sci U S A, 2001 98:5981–85). A large portion of this work uses chemical functions such as alkylated pyridinium derivatives, which are known to be toxic to mammalian cells. The antibiotic ciprofloxacin has been grafted into a degradable polymer backbone (G. L. Y. Woo, et al., Biomaterials 2000 21:1235–1246). The activity of this material relies on cleavage of the active component from the polymer backbone.
Anti-infective vinyl copolymers, wherein monomers with hydrophobic and hydrophilic side chains have been randomly polymerized to produce polymers with amphiphilic properties, have also been described recently W. H. Mandeville III et al. (U.S. Pat. No. 6,034,129). These materials are produced by polymerization of hydrophobic and hydrophilic acrylate monomers. Alternately, the hydrophobic side chain is derived from a styrene derivative which is copolymerized with a hydrophilic acrylate monomer wherein an ionic group is linked to the carboxylic acid. These polymers, however, have relatively random arrangements of polar and nonpolar groups and are not facially amphiphilic as defined herein.
An alternative method to make amphiphilic polymers is to produce block copolymers comprised of hydrophobic blocks (A) and hydrophilic blocks (B), commonly polypropyleneoxy and polyethylenoxy segments respectively, into A-B, A-B-A or similar copolymers. These copolymers also are not facially amphiphilic as defined herein.